A surface-emitting laser (VCSEL: Vertical Cavity Surface Emitting LASER) is a semiconductor laser for emitting light in a vertical direction with respect to a surface of a substrate, and has features of being at a low price, a low electrical power consumption, compact, having a high performance, and being readily integrated two-dimensionally, as compared with an edge-emitting-type semiconductor laser.
A surface-emitting laser has a resonator area including an active layer and a resonator structure composed of an upper reflector and a lower reflector above and below the resonator area, respectively (for example, Japanese Patent Application Publication No 2008-53353). Hence, a resonator area is formed with a predetermined optical thickness in such a manner that light with a wavelength λ oscillates in the resonator area in order to obtain light with an oscillation wavelength λ. An upper reflector and a lower reflector are formed by alternately laminating and forming materials with different refractive indices, namely, a low refractive index material and a high refractive index material, and formed in such a manner that optical film thicknesses of the low refractive index material and high refractive index material are λ/4 in order to obtain a high reflectance at a wavelength λ.
Furthermore, forming elements for different wavelengths in a chip is also disclosed (for example, Japanese Patent No. 2751814, Japanese Patent Application Publication No. 2000-058958, Japanese Patent Application Publication No. 11-330631, and Japanese Patent Application Publication No. 2008-283129). It may be possible to form such a multi-wavelength surface-emitting laser element by forming a wavelength adjustment layer with a structure formed by alternately laminating two materials for different etching fluids on a resonator area of the surface-emitting laser element and by removing such a wavelength adjustment layer one-by-one for each surface-emitting laser by means of wet-etching to change a thickness of the wavelength adjustment layer.
Meanwhile, there is an atomic clock (atomic oscillator) as a clock for timing extremely accurate time, and a technique for miniaturizing such an atomic clock, etc., is studied. An atomic clock is an oscillator based on an amount of transition energy of an electron constituting an alkali metal atom, etc., and in particular, it may be possible to a very precise value of transition energy of an electron in an alkali metal atom on a condition of no disturbance whereby it may be possible to obtain a frequency stability at several orders of magnitude higher than a quartz oscillator.
There are some types of such an atomic clock, and among those, a frequency stability of a Coherent Population Trapping (CPT)—type atomic clock is about three orders of magnitude higher than a conventional quartz oscillator, wherein it may also be possible to expect a very compact type and very low electric power consumption (for example, Applied Physics Letters, Vol. 85, pp. 1460-1462 (2004), Comprehensive Microsystems, vol. 3, pp. 571-612, and Japanese Patent Application Publication No. 2009-188598).
A CPT-type atomic clock has a laser element, a cell enclosing an alkali metal, and a light receiving element for light-receiving laser light transmitted through the cell, wherein laser light is modulated and two transitions of en electron in an alkali metal atom are simultaneously attained by sideband wavelengths occurring at both sides of a carrier wave at a particular wavelength to conduct excitation thereof. Transition energy for such transition is invariable, and when a sideband wavelength of laser light coincides with a wavelength corresponding to transition energy, a transparency increasing phenomenon occurs in which a light absorption rate of an alkali metal is lowered. Thus, such an atomic clock is characterized in that a wavelength of a carrier wave is adjusted to lower a light absorption rate of an alkali metal and a signal detected by a light receiving element is fed back to a modulator so that a modulation frequency of laser light from a laser element is adjusted by the modulator. Additionally, in such an atomic clock, laser light emitted from a laser element irradiates a cell enclosing an alkali metal through a collimator and a λ/4 wave plate.
For a light source for such a very compact type atomic clock, a compact surface emitting laser with a very low electric power consumption and a high wavelength quality is suitable and it is desired that a precision of a wavelength of a carrier wave is in ±1 nm with respect to a particular wavelength (for example, Proc. of SPIE Vol. 6132 613208-1 (2006)).
Meanwhile, when a surface-emitting laser element is used for an atomic clock, it may be necessary to provide a narrow wavelength interval (5 nm) for each surface-emitting laser. Hence, a wavelength adjustment layer is formed on a resonator area of a surface-emitting laser, and accordingly, when such a surface-emitting laser with a narrow wavelength interval is formed, it may be necessary to form a film in such a manner that a thickness of each film in the wavelength adjustment layer is very thin. However, it may be difficult to form a film in such a manner that a thickness of each film for forming a wavelength adjustment layer is extremely thin and uniform, due to a dispersion of a growth rate, an irregularity in a film thickness distribution, etc., at time of forming of a semiconductor layer.
Specifically, as indicated in Japanese Patent No. 2751814, when a wavelength adjustment layer is formed on a resonator area and when an interval of an oscillating wavelength is intended to be 5 nm or less, it may be necessary for a film thickness of a wavelength adjustment layer to be 1.2 nm or less but it may be extremely difficult for a current technique of crystal growth of a compound semiconductor to control such a thin film thickness. Thus, even if a film thickness is changed slightly, an oscillation wavelength may be influenced thereby.